There is a high-oscillation type sensor for detecting passage of a body in which a detecting coil is provided at the surrounding of a portion of the sensor through which a body passes and which detects the passage of a body based on a change in the inductance of the detection coil caused by the passage of the body. However, in this case the material of the body to be detected is limited to a metal and the passage of a nonmetallic body such as those made of plastics or wood cannot be detected. Accordingly, it comes to ones mind to detect the passage of a body irrespective of the material by using an electrostatic sensor of an electrostatic capacitance type.
For example, a through hole 2 that is passable by a body is formed in a box-like case 1 as shown in FIG. 5. At the inside of the case 1 a sensor electrode 3 and a ground electrode 4 are installed in an opposed state apart from each other by a predetermined distance such that the body can pass therebetween through the through hole 2. Along with them a sensor circuit board 5 is disposed at the inner portion of the case 1 and the passage of the body is detected by a detecting circuit of the sensor circuit board 5 based on a change in an electrostatic capacitance between the sensor electrode 3 and the ground electrode 4.
However, in the above-mentioned electrostatic sensor the sensor electrode 3 and the ground electrode 4 are opposed and apart from each other by a predetermined distance such that the body is allowed to pass therebetween through the through hole 2. Therefore, in view of the structure a wide effective area of the sensor electrode 3 cannot be provided and accordingly, the detectable distance is short. Especially, when the body is small, or the like, the passage of the body cannot be detected. Further, the sensor electrode 3 is likely to operate erroneously by being influenced by outside noises and the erroneous operation by outside noises must be prevented by providing a special member around the sensor electrode 3.